pygobject3: fix install for osx-included python

PYTHON env var would not be set for python2 if python2 was not installed from homebrew, and pygobject3's configure script would pick python3 both times. The osx-included python wouldn't get a pygobject3 module.

It's sufficient to just delete the `if Formula[python].installed?` but I like to see why it's building pygobject3 twice (and/or which one failed).

I'm not familiar with the best ways to do things with homebrew or ruby in general, so consider this a bug report with a work-around proof-of-concept included.